The invention relates generally to musical instruments, and more particularly to polyphonic electronic musical instruments using a substantially numerical method. The instruments in question are e.g. electronic organs, electronic accordions or any other instrument, with or without a keyboard, for synthetically producing musical sounds by electronic actuating means.
In prior-art polyphonic instruments, the sounds are produced by sets of oscillators associated with filter and shaping circuits for producing sinusoidal sounds at the fundamental frequency of the played note, together with the various harmonics in the sound of the note as produced by the instrument which is to be imitated. The oscillator outputs are mixed, with suitable amplitude weighting to obtain a complex wave form. Good results are obtained only if there is a large number of oscillators and of filter and shaping circuits. Consequently, the number of electric contacts associated with each key must also be large and the wiring of the circuits and contacts is complex. It is also difficult to obtain a complex wave form which is identical for each played note.
Since the instrument does not imitate only a single conventional instrument but has to simulate a number of sets of instruments preselected by switches, numerous different filter and weighting circuits are required together with numerous set switches, which further complicates the wiring.
After the synthesis has been made, the attack, sustain and extinction periods of each note have to be shaped so as to simulate the mechanical delay inherent in the beginning or end of a sound produced e.g. by an organ pipe and bellows, or the sudden attack of the high-rank harmonics in the case of a piano, the subsequent extinction being variable for each harmonic of the sound. Usually, these attack and extinction coefficients are produced by charging and discharging a capacitor providing a voltage whih increases or decreases in logarithmic manner. In that case, the amplitude of the resulting note has to follow the variations in the increasing or decreasing voltage. This method limits the choice of the attack and extinction characteristics, which differ in both time and frequency in the case of practically all the instruments which it is desired to imitate. Furthermore, the use of percussion circuits for obtaining these effects results in considerable extra complexity in wiring and the circuits, particularly when a polyphonic effect is required.
In some prior-art electronic organs, numerical circuits are used to produce sounds. The waves to be reproduced are stored in the form of numerical samples which are read at variable speeds to reproduce all the notes played by the instrument. A number of wave forms can be stored in a number of stores to simulate a number of sets of different instruments.
In other prior-art organs, samples of a sinusoidal function are stored instead of the complex wave form to be reproduced by the instrument. In that case, the complex sound of an instrument must be obtained by producing samples of the fundamental note and of the harmonics and adding them at suitable amplitudes before converting them to analog signals.
Hitherto, these numerical methods have been difficult to apply to truly polyphonic instruments and, in order to pay several notes simultaneously, it has been necessary to multiply the number of circuits, since these can play only a single note at once. Consequently, control of the circuits by the manual keys or pedals becomes a complex operation requiring numerous circuits and complex, expensive wiring. Furthermore, in order to obtain the various kinds of sound, the number of stores and amplitude control circuits has to be multiplied by the number of different notes which can be played simultaneously.